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DP IB Chemistry: SL

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Home / IB / Chemistry: SL / DP / Topic Questions / 5. Energetics / Thermochemistry / 5.2 Hess's Law / Structured Questions: Paper 2


5.2 Hess's Law

Question 1a

Marks: 2
a)
Vanadium is commonly found in different ores such as magnetite, vanadinite and patronite. The vanadium is commonly extracted from these ores by reduction and displacement.

Vanadium can be extracted by the reduction of vanadium pentoxide, V2O5, with calcium at high temperatures, according to the following equation.

V2O5 (s) + 5Ca (s) → 2V (s) + 5CaO (s)

The enthalpy of formation of vanadium pentoxide is -1560 kJ mol-1 and the standard enthalpy change for the reaction is -1615 kJ mol-1


Construct a Hess’s Law cycle for this reaction.

[2]

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    Question 1b

    Marks: 3
    b)
    Use the data in part a) to calculate the enthalpy of formation, ΔHf, of calcium oxide in kJ mol-1
    [3]
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      Question 1c

      Marks: 2
      c)
      Define standard enthalpy of neutralisation, ΔHneut
      [2]

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        Question 2a

        Marks: 3
        a)
        The compound diborane, B2H6, is used as a rocket fuel. The equation for the combustion of diborane is shown below.

        B2H6 (g) + 3O2 (g) → B2O3 (s) + 3H2O (l)

        Calculate the standard enthalpy change of this reaction using the following data

        I. 2B (s) + 3H2 (g) → B2H6 (g)          ΔH = 36 kJ mol-1
        II. H2 (g) + ½O2 (g) → H2O (l)          ΔH = -286 kJ mol-1
        III. 2B (s) + 1½O2 (g) → B2O3 (s)       ΔH = -1274 kJ mol-1
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          Question 2b

          Marks: 1
          b)
          Ethyne, C2H2, is a useful gas as it gives a high temperature flame when burnt with oxygen. State the equation for the combustion of ethyne gas.
          [1]

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            Question 2c

            Marks: 3
            c)
            Use your answer to part b) to construct a Hess's Law cycle for the combustion of ethyne gas.
            [3]
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              Question 2d

              Marks: 3
              d)
              Use sections 12 and 13 in the data booklet to determine the enthalpy of formation, ΔHf, of ethyne gas. 
              [3]

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                Question 3a

                Marks: 3
                a)
                Coal gasification converts coal into a combustible mixture of carbon monoxide and hydrogen known as coal gas, in a gasifier. 

                H2O (l) + C (s) → CO (g) + H2 (g) 

                Using the following equations, calculate the enthalpy change of reaction, ΔHr, in kJ for cola gasification. 

                I. 2C (s) + O2 (g) → 2CO (g)            ΔH = -222 kJ
                II. 2H(g) + O2 (g) → 2H2O (g)        ΔH = -484 kJ
                III. H2O (l) → H2O (g)                       ΔH = +44 kJ
                [3]
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                  Question 3b

                  Marks: 3
                  b)
                  This coal gas can be used as a fuel as the following equation shows.

                  CO (g) + H2 (g) + O2 (g) → CO2 (g) + H2O (g) 

                  Calculation the enthalpy change of reaction, ΔHr, in kJ for this combustion reaction from the following equations.

                  I. 2C (s) + O2 (g) → 2CO (g)       Δ= -222 kJ
                  II. C (s) + O2 (g) → CO2 (g)         Δ= -394 kJ
                  III. 2H2 (g) + O2 (g) → 2H2O (g) Δ= -484 kJ
                  [3]
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                    Question 3c

                    Marks: 2
                    c)
                    Blending amounts of alternative fuel with conventional fuel is one way to replace petroleum. A fuel blend of 51% to 83% ethanol and the remaining being gasoline is known as E85.

                    If the fuel blend is vaporised before combustion, predict whether the amount of energy released would be greater, less or the same. Explain your answer.
                    [2]
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                      Key Concepts
                      Bond enthalpy

                      Question 3d

                      Marks: 3
                      d)      Use sections 6 and 13 of the Data booklet to calculate the following.

                      i)
                      The amount, in moles, of ethanol in 1 kg of E85 containing 60% ethanol.

                      [2]

                      ii)
                      The energy released, in kJ, by ethanol if 1 kg of E85 is burnt.

                      [1]

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                        Question 4a

                        Marks: 3
                        a)
                        Strontium salts have a number of applications such as fireworks, flares, glow in the dark paint and toothpaste for sensitive teeth. The strontium required for these salts can be extracted from the ore strontia, SrO, by displacement with powdered aluminium in a vacuum.

                        i)
                        Write a balanced symbol equation, including state symbols, for the reaction of strontia with aluminium.
                        [2]

                        ii)
                        State the role of the aluminium in this reaction.
                        [1]
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                          Question 4b

                          Marks: 3
                          b)
                          The standard enthalpy change for this extraction of strontium is 99.3 kJ mol-1 and the standard enthalpy of formation of aluminium oxide is -1676.7 kJ mol-1


                          Use this information to calculate the standard enthalpy of formation, ΔHf, in kJ mol-1 of strontia.

                          [3]

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                            Question 4c

                            Marks: 3
                            c)
                            Manganese is too brittle for use as a pure metal, so it is often alloyed with other metals. Manganese is used in steel to increase the strength and resistance to wear. Manganese steel (13% Mn) is extremely strong and used for railway tracks, safes and prison bars. Alloys of 1.5% manganese with aluminium are used to make drinks cans due to the improved corrosion resistance of the alloy.

                            Manganese is extracted from different ores by reduction with carbon monoxide.

                            Mn2O3 (s) + 3CO (g) → 2Mn (s) + 3CO2 (g)

                             

                            The enthalpy of formation, ΔHf, of Mn2O(s) is −971 kJ mol-1. Use this information and section 12 of the data booklet to calculate the enthalpy change of reaction, ΔHr, in kJ mol-1
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                              Question 4d

                              Marks: 3
                              d)
                              The reaction in part c) reaches equilibrium at high temperatures.

                              Use your answer to part c) to explain how temperature can be altered to increase the yield of
                              the reaction and explain the effect that this would have on the rate of reaction.
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